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A single LED on an dual DC supply?
A single LED on an dual DC supply?
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Old 26th November 2012, 09:45 AM   #11
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by sofaspud View Post
Multicolor LEDs are usually available as both CC and CA. The cathode-to-anode types seem to be only 2-lead parts. So I can't think of an elegant way to do this. My drawing of Robert's suggestion will work, and here's an alternate way adding a 1N4148 diode that functions similarly. Otherwise, DF96's suggestion of some type of logic switching seems like the next step up.
The circuit on the left will not work. The one on the right will, but it has a flaw: both LEDs will light as soon as the positive supply is present.
Here are two examples: the left one is based on common cathode LEDs, and they will light if and only if their respective supply is present.

On the right is the two-terminal version: the AC voltage from the transformer serves as a switching source.
The LEDs will light only if the AC voltage AND their respective supply is present.
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File Type: gif LEDex.gif (9.6 KB, 165 views)
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Old 26th November 2012, 10:28 AM   #12
sofaspud is offline sofaspud  United States
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Quote:
The circuit on the left will not work. The one on the right will, but it has a flaw: both LEDs will light as soon as the positive supply is present.
I'm not seeing why the first circuit wouldn't work. Is the voltage difference too great?
For the second circuit, if the negative line is open why would that LED light? Something downstream? I had it figured that the change in color would be the fault signal. The flaw was that the third condition only caused a brightness change, not a color change.
You have interesting solutions. I had considered a variation of the left circuit, then began to wonder if the different Vf's of a RGB LED could be utilized to show three status colors.
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Old 26th November 2012, 11:04 AM   #13
Robert Kesh is offline Robert Kesh  Wallis & Futuna
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Quote:
Originally Posted by sofaspud View Post
I'm not seeing why the first circuit wouldn't work. Is the voltage difference too great?
You didn't put any resistors in series with the individual LEDs, as I suggested in my original post. Any extra components are not necessary however.
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Old 26th November 2012, 11:27 AM   #14
sofaspud is offline sofaspud  United States
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I drew individual LEDs only for clarity. It's supposed to be a single common-cathode bicolor, with the LED current flowing through "R2". I'm not seeing how putting individual resistors on the anode side makes all the difference.
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Old 26th November 2012, 11:32 AM   #15
Robert Kesh is offline Robert Kesh  Wallis & Futuna
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Quote:
Originally Posted by sofaspud View Post
I'm not seeing how putting individual resistors on the anode side makes all the difference.
In your circuit, what's the voltage drop across each LED?
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Old 26th November 2012, 11:36 AM   #16
sofaspud is offline sofaspud  United States
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It depends on the LED. Let's say 2 volts each.

edit: Ahhh...OK. That's what I meant by the voltage difference in post #12. That does throw things into a mess.
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Last edited by sofaspud; 26th November 2012 at 11:43 AM. Reason: eureka
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Old 26th November 2012, 11:42 AM   #17
Robert Kesh is offline Robert Kesh  Wallis & Futuna
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Last edited by Robert Kesh; 26th November 2012 at 12:05 PM.
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Old 26th November 2012, 12:23 PM   #18
Robert Kesh is offline Robert Kesh  Wallis & Futuna
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Only problem is the circuit will draw a lot more current than normal LEDs and one line dying will affect the brightness of the other LED a bit.
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File Type: jpg common cathode.jpg (10.9 KB, 150 views)
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Old 26th November 2012, 10:00 PM   #19
teleman is offline teleman  Norway
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Just my 2 cents worth...will any of these combinations help?

The 1st LED is a 2 anode + 2 cathode device.....forgot where i found it! Is there anway the 3rd circuit could be modified for a +/- supply?
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File Type: jpg 2 anode + 2 cathode LED.jpg (32.8 KB, 135 views)
File Type: gif Bicolor-comon cathode.gif (6.8 KB, 135 views)
File Type: jpg led-display-for-on-off-battery-indicator.jpg (29.3 KB, 106 views)

Last edited by teleman; 26th November 2012 at 10:15 PM.
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Old 26th November 2012, 10:14 PM   #20
sofaspud is offline sofaspud  United States
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Only those that fit into one of the previous submissions.
I can add another: two individual LEDs side by side connected with heatshrink to a light pipe on the front panel. That should provide a 4-state psu status.
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